Tunable control of pozzolan-lime cement compositions

ABSTRACT

A variety of cement compositions and methods of using cement compositions in subterranean formations. A method of formulating a cement composition may comprise: measuring surface area of a group of pozzolans, wherein the smallest measured surface area of the group of pozzolans and the largest measured surface area of the group of pozzolans vary by about 50% or greater. The method may further comprise selecting one or more pozzolans from the group of pozzolans. The method may further comprise adding components comprising lime and water to the selected one or more pozzolans to faun a cement composition.

BACKGROUND

The present embodiments relate to cement compositions and, in particularembodiments, to controlling the properties of cement compositionscomprising a pozzolan and lime.

Cement compositions may be used in a variety of subterranean operations.For example, in subterranean well construction, a pipe string (e.g.,casing, liners, expandable tubulars, etc.) may be run into a wellboreand cemented in place. The process of cementing the pipe string in placeis commonly referred to as “primary cementing.” In a typical primarycementing method, to cement composition may be pumped into an annulusbetween the walls of the wellbore and the exterior surface of the pipestring disposed therein. The cement composition may set in the annularspace, thereby forming an annular sheath of hardened, substantiallyimpermeable cement (i.e., a cement sheath) that may support and positionthe pipe string in the wellbore and may bond the exterior surface of thepipe string to the subterranean formation. Among other things, thecement sheath surrounding the pipe string prevents the migration offluids in the annulus and protects the pipe string from corrosion.Cement compositions may also be used in remedial cementing methods toseal cracks or holes in pipe strings or cement sheaths, to seal highlypermeable formation zones or fractures, or to place a cement plug andthe like.

A broad variety of cement compositions have been used in subterraneancementing operations. In some instances, set-delayed cement compositionshave been used. Set-delayed cement compositions are characterized bybeing capable of remaining in a pumpable fluid state for at least aboutone day (e.g., about 7 days, about 2 weeks, about 2 years or more) atroom temperature (e.g., about 80° F.) in quiescent storage. When desiredfor use, the set-delayed cement compositions should be capable ofactivation and consequently develop reasonable compressive strengths.For example, a cement set activator may be added to a set-delayed cementcomposition to induce the composition to set into a hardened mass. Amongother things, set-delayed cement compositions may be suitable for use inwellbore applications such as applications where it is desirable toprepare the cement composition in advance. This may allow the cementcomposition to be stored prior to use. In addition, this may allow thecement composition to be prepared at a convenient location beforetransportation to the job site. Accordingly, capital expenditures may bereduced due to a reduction in the need for on-site bulk storage andmixing equipment. This may be particularly useful for offshore cementingoperations where space onboard the vessels may be limited.

While cement compositions and set-delayed cement compositions have beendeveloped heretofore, problems may exist when formulating thesecompositions for specific applications. For example, a composition withlow reactivity may be desired such that the composition may maintain alow viscosity for applications requiring the cement composition toremain in a pumpable fluid state for as long period. However, such lowreactivity fluids may not possess sufficient compressive strength whenset to actually be suitable compositions for their intended use.Typically, cement compositions may use additives to enhance cementproperties (e.g., compressive strength). However, additives may createadditional costs as well as compatibility issues with other fluids orwith the formation itself. Therefore, it may be desirable to adjust theproperties of a cement composition while limiting the need to adjust thecompositional makeup of the cement composition.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of sonic of the embodiments ofthe present methods and compositions, and should not be used to limit ordefine the methods or compositions.

FIG. 1 illustrates a system for preparation and delivery of a cementcomposition to a wellbore in accordance with certain embodiments.

FIG. 2 illustrates surface equipment that may be used in placement of acement composition in a wellbore in accordance with certain embodiments.

FIG. 3 illustrates placement of a cement composition into a wellboreannulus in accordance with certain embodiments.

DETAILED DESCRIPTION

The present embodiments relate to cement compositions and, in particularembodiments, to controlling the properties of cement compositionscomprising a pozzolan and lime. For example, properties of the cementcomposition may be tuned for a specific application. Embodiments maycomprise selection of a pozzolan based on surface area and/or particlesize and further optional embodiments may comprise the blending of twoor more pozzolans to control the properties of a cement composition

Embodiments of the cement compositions may generally comprise water, apozzolan, and lime. Optionally, the cement compositions may furthercomprise a dispersant and/or a retarder. Embodiments of the cementcompositions comprising a retarder may be set-delayed such that they arecapable of remaining in a pumpable fluid state for an extended period oftime. For example, the set-delayed cement compositions may remain in apumpable fluid state for at least about 1 day or longer. The cementcompositions may be suitable for a number of cementing operations; theymay be particularly suitable for use in subterranean formations havingrelatively low bottom hole static temperatures, e.g., temperatures ofabout 200 or less or ranging from about 100° F. to about 200° F. Inalternative embodiments, the cement compositions may be used insubterranean formations having bottom hole static temperatures up to450° F. or higher.

The water used in embodiments may be from any source provided that itdoes not contain an excess of compounds that may undesirably affectother components in the cement compositions. For example, a cementcomposition may comprise fresh water or salt water. Salt water generallymay include one or more dissolved salts therein and may be saturated orunsaturated as desired for a particular application. Seawater or brinesmay be suitable for use in embodiments. Further, the water may bepresent in an amount sufficient to form a pumpable slurry. In certainembodiments, the water may be present in the cement compositions in anamount in the range of from about 33% to about. 200% by weight of thepozzolan. In certain embodiments, the water may be present in the cementcompositions in an amount in the range of from about 35% to about 70% byweight of the pozzolan. With the benefit of this disclosure one ofordinary skill in the art will recognize the appropriate amount of waterfor a chosen application.

Embodiments of the cement compositions may comprise a pozzolan. Avariety of different pozzolans may be suitable for use in embodiments.Example embodiments comprising a pozzolan may comprise fly ash, silicafume, metakaolin, a natural pozzolan (e.g., pumice), or combinationsthereof. In some embodiments, the cementitious components present in thecement composition may consist essentially of the pozzolan. For example,the cementitious components may primarily comprise the pozzolan withoutany additional cementitious components (e.g., Portland cement) thathydraulically set in the presence of water.

An example of a suitable pozzolan may comprise fly ash. A variety of flyash may be suitable for particular embodiments, including fly ashclassified as Class C and Class F fly ash according to the AmericanPetroleum institute, API Specification for Materials and Testing forWell Cements, API Specification 10, Fifth Ed., July 1, 1990. Class C flyash comprises both silica and lime, so it may set to form a hardenedmass upon mixing with water. Class F fly ash generally does not containa sufficient amount of lime to induce a cementitious reaction,therefore, an additional source of calcium ions is necessary for acement composition comprising Class F fly ash. In some embodiments, limemay be mixed with Class F fly ash in an amount in the range of about0.1% to about 100% by weight of the fly ash. In some instances, the limemay be hydrated lime. Suitable examples of fly ash include, but are notlimited to, POZMIX® A cement additive, commercially available fromHalliburton Energy Services, Inc., Houston, Tex.

An example of a suitable pozzolan may comprise metakaolin. Generally,metakaolin is a white pozzolan that may be prepared by heating kaolinclay to temperatures in the range of about 600 to about 800° C.

An example of a suitable pozzolan may comprise a natural pozzolan.Natural pozzolans are generally present on the Earth's surface and setand harden in the presence of hydrated lime and water. For example,pumice is a natural pozzolan. Pumice is a volcanic rock that may exhibitcementitious properties. An example of a suitable pumice is availablefrom Hess Pumice Products, Inc., Malad, Id., as DS-325 lightweightaggregate. Pumice Example embodiments comprising a natural pozzolan maycomprise pumice, diatomaceous earth, volcanic ash, opaline shale, tuff,and combinations thereof. The natural pozzolans may be ground orunderground.

Generally, the pozzolan may have any particle size distribution asdesired for a particular application. As used herein, “particle size”refers to volume surface mean diameter (“D_(S)”) which is related to thespecific surface area. Volume surface mean diameter may be defined bythe following formula: D_(S)=6/(Φ_(S)A_(w)ρ_(p)) where Φ_(S)=sphericityA_(w)=Specific surface area and ρ_(p)=Particle density. It should beunderstood that the particle size may vary based on the measurementtechnique, sample preparation, and sample conditions (e.g., temperature,concentration, etc.). In certain embodiments, the pozzolan may have ad50 particle size distribution in a range of from about 1 micron toabout 200 microns. The d50 values may be measured by particle sizeanalyzers such as those manufactured by Malvern Instruments,Worcestershire, United Kingdom. In specific embodiments, the pozzolanmay have a d50 particle size distribution in a range of from about 1micron to about 200 microns, from about 5 microns to about 100 microns,or from about 10 microns to about 25 microns. In a particularembodiment, the pozzolan may have a d50 particle size distribution ofabout 15 microns or less. An example of a suitable pozzolan is availablefrom Hess Pumice Products, Inc., Malad, Id., which is a DS-325lightweight aggregate pumice, having a d50 particle size distribution ofabout 12-17 microns.

Generally, the pozzolan may have any size surface area as desired for aparticular application. As used herein, “surface area” is defined as thetotal area of the particle's faces and curved surfaces. The specificsurface area may be calculated by the Brunauer-Emmett-Teller methodologyof measuring the gas adsorption on a particle surface. In certainembodiments, the pozzolan may have a surface area in a range of fromabout 1 m²/g to about 500 m²/g, or even greater. The surface area valuesmay be measured by a Quantachrome NOVA® 2200e surface area analyzer suchas those manufactured by Quantachrome Instruments, Boynton Beach, Fla.In specific embodiments, the pozzolan may have to surface area in arange of from about 1 m²/g to about 20 /²/g, from about from about 5m²/g to about 20 m²/g, or from about 10 m²/g to about 15 m²/g. In aparticular embodiment, the pozzolan may have a surface area of about 20m²/g or less. An example of a suitable pozzolan is available from HessPumice Products, Inc., Malad, Id., which is a DS-325 lightweightaggregate pumice, having a surface area distribution of about 20 m²/g orless.

It should be appreciated that particle sizes too small may havemixability problems while particle sizes too large may not beeffectively suspended in the cement compositions. Similarly, surfaceareas too large may be too reactive and prematurely gel or set, whilesurface areas too small may be too unreactive and not set quick enoughor provide enough compressive strength. Therefore, embodiments compriseselecting a pozzolan based on the dimensions of particle size andsurface area such that the shelf life, theology, and compressivestrength may be tuned to provide a cement composition with the specificproperties desired for a particular application.

In embodiments, the pozzolan may comprise two or more pozzolans withdifferent surface areas and/or different particle sizes. For example,the pozzolans may comprise pozzolans with particles sizes varying by asmuch as 50% or more, such that the pozzolan particles of the two or morepozzolans may have particles sizes that vary by about 10%, about 20%,about 30%, about 40%, about 50%, or more. As another example, thepozzolans may comprise particles with surface areas varying by as muchas 50% or more, such that the pozzolan particles of the ts,vo or morepozzolans may have surface areas that vary by about 10%, about 20%,about 30%, about 40%, about 50%, or more. In particular embodiments, twoor more pozzolans may be used having particle sizes that vary about 10%or less, but have surface areas that vary by about 10% or more. By wayof example, the surfaces areas of the two or more pozzolans may varyabout 10%, about 20%, about 30% about 40%, about 50%, or more while theparticles sizes vary by about 10%, about 5%, or less. The two or morepozzolans may be the same type of pozzolan (e.g., pumice), but have asurface area that varies. In embodiments, requiring particles with aspecific distribution of particles sizes and/or surface areas, it maytherefore be necessary to select and isolate pozzolan particles.

Embodiments of the present technique may comprise measuring the particlesize and surface areas of a group of pozzolans and/or sorting thepozzolans according to those measurements. The group of pozzolans maycomprise pozzolans of the same type but with different particle sizeand/or surface areas. The particle size and surface areas may bemeasured by any sufficient means, such as those techniques discussedherein. Once the particle size and surface area have been measured, thepozzolans may be sorted according to those measurements. Sorting of thepozzolans may comprise isolating and the organizing the pozzolans intogroups or categories that are specific to a predetermined range ofparticle size and/or surface area distributions. Alternatively, thesorting may comprise filtration. In embodiments, the pozzolans may befiltered by any sufficient means such that the pozzolans are sorted viafiltration into two or more groups based on particle size, surface area,or both. Isolating and sorting pozzolan particles based on particle sizeand/or surface area may allow for tuning of pozzolan properties such asshelf life, theology, compressive strength, and overall reactivity. Oneof ordinary skill in the art, with the benefit of this disclosure,should be able to isolate and select a particle size and/or surface areafor the pozzolan pozzolans) suitable for a chosen application.

In embodiments, the pozzolan may comprise a blend of two or morepozzolans. By way of example, one or more dry pozzolans may be blendedwith one or more dry pozzolans. Alternatively, one or more wet pozzolansmay be blended with one or more dry pozzolans. Further, one or more wetpozzolans may be blended with one or more wet pozzolans. The blend ofpozzolans may comprise a blend of two or more pozzolans of differenttypes e.g., a blend of fly ash and pumice, and/or the blend may comprisea blend of two or more pozzolans of the same type (e.g., pumice) withparticles of different surface areas and/or particle sizes, e.g., ablend of pumice comprising a surface area of 18 m²/g with a pumicecomprising a surface area of 12 m²/g. Blending of a pozzolan based ontype and/or particle size and/or surface area may allow for theformulation of a pozzolan slurry comprising a shelf life, rheology,compressive strength, and overall reactivity suitable for a specificapplication. For example, a pozzolan comprising a high surface area maybe blended with a sufficient amount of a pozzolan comprising a lowsurface area such that the overall blend has a long shelf life andmaintains a low viscosity but also has a sufficient compressive strengthupon setting. One of ordinary skill in the art, with the benefit of thisdisclosure, should be able to blend two or more pozzolans to create acement composition suitable for a chosen application.

In embodiments, the pozzolans may be blended using any sufficientmechanism Or technique. For example, the pozzolans may be blended by abatch or continuous mixer (e.g., rotary, ribbon, paddle and plow, andthe like). The process of blending the pozzolans may require the mixingequipment to be controlled in such, a way so as to not induce furtheralterations to the particle size or surface area of the pozzolans.Although the pozzolans may be blended by any such mechanism or techniquesufficient to produce a blended composition, mechanisms or techniquesthat minimize alterations to the particle size or surface areas of theparticles are preferred. The pozzolans may be blended in any ratio toachieve a specific range of properties necessary for a specificapplication. For example, pozzolans of varying, surface areas andparticle sizes may be blended such that the blended pozzolan is capableof remaining in a pumpable fluid state for greater than 5 days withminimal to no impact on the compressive strength of the pozzolan once ithas set. As a further example, the ratio of one pozzolan to another maybe about 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20, orabout 90:10. One of ordinary skill in the art, with the benefit of thisdisclosure, should be able to blend two or more pozzolans to create acement composition suitable for a chosen application.

Embodiments of the cement compositions may comprise lime. Further, thelime in some embodiments may comprise hydrated lime. As used herein, theterm “hydrated lime” will be understood to mean calcium hydroxide. Insome embodiments, the lime may be provided as quicklime (calcium oxide)which hydrates when mixed with water to form a hydrated lime. The limemay be included in embodiments of the cement compositions, for example,to form a hydraulic composition with the pozzolan. For example, the limemay be included in a pozzolan-to-lime weight ratio of about 10:1 toabout 1;1 or a ratio of about 3:1 to about 5:1. Where present, the limemay be included in the cement compositions in an amount in the range offrom about 10% to about 100% by weight of the pozzolan, for example. Insome embodiments, the lime may be present in an amount ranging betweenany of and/or including any of about 10%, about 20%, about 40%, about60%, about 80%, or about 100% by weight of the pozzolan. One of ordinaryskill in the art, with the benefit of this disclosure, will recognizethe appropriate amount of lime to include for a chosen application.

Embodiments of the cement compositions may comprise a set retarder. Abroad variety of set retarders may be suitable for use in the cementcompositions. For example, the set retarder may comprise phosphonicacids, such as ethylenediamine tetra(methylene phosphonic acid),diethylenetriamine penta(methylene phosphonic acid), etc.;lignosulfonates, such as sodium lignosulfonate, calcium lignosulfonate,etc.; salts such as stannous sulfite, lead acetate, monobasic calciumphosphate, organic acids, such as citric acid, tartaric acid, etc.;cellulose derivatives such as hydroxyl ethyl cellulose (HEC) andcarboxymethyl hydroxyethyl cellulose (CMHEC); synthetic co- orterpolymers comprising sulfonate and carboxylic acid groups such assulfonate-functionalized acrylamide-acrylic acid copolymers; boratecompounds such as alkali borates, sodium metaborate, sodium tetraborate,potassium pentaborate; derivatives thereof, or mixtures thereof.Examples of suitable set retarders include, among others, phosphonicacid derivatives. One example of a suitable set retarder is MicroMatrix® cement retarder, available from Halliburton Energy Services,Inc. Generally, the set retarder may be present in the cementcompositions in an amount sufficient to delay the setting for a desiredtime. In some embodiments, the set retarder may be present in the cementcompositions in an amount in the range of from about 0,01% to about 10%by weight of the pozzolan. In specific embodiments, the set retarder maybe present in an amount ranging between any of and/or including any ofabout 0.01%, about 0.1%, about 1%, about 2%. about 4%, about 6%, about8%, or about 10% by weight of the pozzolan. One of ordinary skill in theart, with the benefit of this disclosure, will recognize the appropriateamount of the set retarder to include for a chosen application.

As previously mentioned, embodiments of the cement compositions mayoptionally comprise a dispersant. Examples of suitable dispersantsinclude, without sulfonated-formaldehyde-based dispersants (e.g.,sulfonated acetone formaldehyde condensate), examples of which mayinclude Daxad® 19 dispersant available from Geo Specialty Chemicals,Ambler, Pa. Other suitable dispersants may be polycarboxylated etherdispersants such as Liquiment® 5581F and Liquiment® 5141, dispersantsavailable from BASF Corporation Houston, Tex.; or Ethacryl™ G dispersantavailable from Coatex, Genay, France. An additional example of asuitable commercially available dispersant is CFR™-3 dispersant,available from Halliburton Energy Services, Inc, Houston, Tex. TheLiquiment® 514L, dispersant may comprise 36% by weight of thepolycarboxylated ether in water. While as variety of dispersants may beused in accordance with embodiments, polycarboxylated ether dispersantsmay be particularly suitable for use in some embodiments. Without beinglimited by theory, it is believed that polycarboxylated etherdispersants may synergistically interact with other optional componentsof the cement composition. For example, it is believed that thepolycarboxylated ether dispersants may react with certain set retarders(e.g., phosphonic acid, derivatives) resulting in formation of a gelthat suspends the pozzolan and lime in the composition for an extendedperiod of time.

In some embodiments, the dispersant may be included in the cementcompositions in an amount in the range of from about 0.01% to about 5%by weight of the pozzolan. In specific embodiments, the dispersant maybe present in an amount ranging between any of and/or including any ofabout 0.01%, about 0.1%, about 0.5%, about 1%, about 2%, about 3%, about4%, or about 5% by weight of the pozzolan. One of ordinary skill in theart, with the benefit of this disclosure, will recognize the appropriateamount of the dispersant to include for a chosen application.

Some embodiments of the cement compositions may comprise silica sourcesin addition to the pozzolan; for example, crystalline silica and/oramorphous silica. Crystalline silica is a powder that may be included inembodiments of the cement compositions, for example, to prevent cementcompressive strength retrogression. Amorphous silica is a powder thatmay be included in embodiments of the cement compositions as alightweight filler and/or to increase cement compressive strength.Amorphous silica is generally a byproduct of a ferrosilicon productionprocess, wherein the amorphous silica may be formed by oxidation andcondensation of gaseous silicon suboxide, SiO, which is formed as anintermediate during the process. An example of a suitable source ofamorphous silica is Silicalite cement additive available fromHalliburton Energy Services, Inc., Houston, Tex. Embodiments comprisingadditional silica sources may utilize the additional silica source asneeded to enhance compressive strength or set times.

Other additives suitable for use in cementing operations also may beincluded in embodiments of the cement compositions. Examples of suchadditives include, but are not limited to: weighting agents, lightweightadditives, gas-generating additives, mechanical-property-enhancingadditives, lost-circulation materials, filtration-control additives,fluid-loss-control additives, defoaming agents, foaming agents,thixotropic additives, and combinations thereof. In embodiments, one ormore of these additives may be added to the cement compositions afterstoring but prior to the placement of a cement composition. A personhaving ordinary skill in the art, with the benefit of this disclosure,should readily be able to determine the type and amount of additiveuseful for a particular application and desired result.

Those of ordinary skill in the art will appreciate that embodiments ofthe cement compositions generally should have a density suitable for aparticular application. By way of example, the cement compositions mayhave a density in the range of from about 4 pounds per gallon (“lb/gar”)to about 20 lb/gal. In certain embodiments, the cement compositions mayhave a density in the range of from about 8 lb/gal to about 17 lb/gal.Embodiments of the cement compositions may be foamed or unframed or maycomprise other means to reduce their densities, such as hollowmicrospheres, low-density elastic beads, or other density-reducingadditives known in the art. In embodiments, weighting additives may beused to increase the density of the cement compositions. Examples ofsuitable weighting additives may include barite, hematite, hausmannite,calcium carbonate, siderite, ilmenite, or combinations thereof. Inparticular embodiments, the weighting additives may have a specificgravity of 3 or greater. Those of ordinary skill in the art, with thebenefit of this disclosure, will recognize the appropriate density for aparticular application.

As previously mentioned, the cement compositions that have been retardedsuch that they comprise a set-delayed state may be capable of remainingin a pumpable fluid state for at least one day (e.g., at least about 1day, about 2 weeks, about 2 years or more) at room temperature (e.g.,about 80° F.) in quiescent storage. For example, the set-delayed cementcompositions may remain in a pumpable fluid state for a period of timefrom about 1 day to about 7 days or more. In some embodiments, theset-delayed cement compositions may remain in a pumpable fluid state forat least about 1 day, about 7 days, about 10 days, about 20 days, about30 days, about 40 days, about 50 days, about 60 days, or longer. Thecompositions may be stored while remaining in a pumpable fluid stateuntil required. Moreover, the compositions may be transported whileremaining in a pumpable fluid state. A fluid is considered to be in apumpable fluid state where the fluid has a consistency of less than 70Bearden units of consistency (“Bc”), as measured on a pressurizedconsistometer in accordance with the procedure for determining cementthickening times set forth in API RP Practice 10B-2, RecommendedPractice for Testing Well Cements, First Edition, July 2005.

As show below in the examples, a pozzolan (e.g., pumice) with a lowersurface area may be used in some embodiments for preparation of aset-delayed cement composition. The lower surface area of the pozzolanmay lessen the tendency of a more reactive pozzolan to gel or otherwiseinteract with other components of the cement composition. In particularembodiments, the pozzolan used in the set-delayed cement composition mayhave a surface of from about 2 m²/g to about 10 m^(2/g) and,alternatively, from about 3 m²/g to about 7 m²/g. A pozzolan having ahigher surface area (e.g., >10 m²/g) may be blended with the lowersurface area pozzolan to provide increased activity in some embodiments.By way of example, a lower surface area pumice may be blended with ahigh surface area pumice.

When desired for use, the retarded and/or set-delayed, cementcomposition embodiments may be activated (e.g., by combination with anactivator or by thermal activation) to set into a hardened mass. Theterm “cement set activator” or “activator”, as used herein, refers to anadditive that activates a set-delayed and/or retarded cement compositionand may also accelerate the setting of the set-delayed and/or retardedcement composition. By way of example, embodiments of the set-delayedcement compositions may be activated, to form a hardened mass in a timeperiod in the range of from about 1 hour to about 12 hours. For example,embodiments of the set-delayed cement compositions may set to form ahardened mass in a time period ranging between any of and/or includingany of about 1 day, about 2 days, about 4 days, about 6 days, about 8days, about 10 days, or about 12 days.

Embodiments may include the addition of a cement set activator toretarded and/or set-delayed cement composition embodiments. Examples ofsuitable cement set activators include, but are not limited to:zeolites, amities such as triethanolamine, diethanolamine; silicatessuch as sodium silicate; zinc formate; calcium acetate; Groups IA and HAhydroxides such as sodium hydroxide, magnesium hydroxide, and calciumhydroxide; monovalent salts such as sodium chloride; divalent salts suchas calcium chloride; nanosilica (i.e., silica having a particle size ofless than or equal to about 100 nanometers); polyphosphates; andcombinations thereof. In some embodiments, a combination of thepolyphosphate and a monovalent salt may be used for activation. Themonovalent salt may be any salt that dissociates to form a monovalentcation, such as sodium and potassium salts. Specific examples ofsuitable monovalent salts include potassium sulfate, and sodium sulfate.A variety of different polyphosphates may be used in combination withthe monovalent salt for activation of the retarded and/or set-delayedcement compositions, including polymeric metaphosphate salts, phosphatesalts, and combinations thereof. Specific examples of polymericmetaphosphate salts that may be used include sodium hexametaphosphate,sodium trimetaphosphate, sodium tetrametaphosphate, sodiumpentametaphosphate, sodium heptametaphosphate, sodium octametaphosphate,and combinations thereof. A specific example of a suitable cement setactivator comprises a combination of sodium sulfate and sodiumhexametaphosphate. In particular embodiments, the activator may beprovided and added to the retarded and/or set-delayed cement compositionas a liquid additive, for example, a liquid additive comprising asmonovalent salt, a polyphosphate, and optionally a dispersant.

The cement set activator should be added to embodiments of the retardedand/or set-delayed cement compositions in an amount sufficient to inducethe set-delayed composition to set into a hardened mass. In certainembodiments, the cement set activator may be added to the retardedand/or set-delayed cement composition in an amount in the range of about0.1% to about 20% by weight of the pozzolan. In specific embodiments,the cement set activator may be present in an amount ranging between anyof and/or including any of about 0.1%, about 1%, about 5%, about 10%,about 15%, or about 20% by weight of the pozzolan. One of ordinary skillin the art, with the benefit of this disclosure, will recognize theappropriate amount of the cement set activator to include for a chosenapplication.

In embodiments, the cement compositions may set to have a desirablecompressive strength after activation. Compressive strength is generallythe capacity of a material or structure to withstand axially directedpushing forces. The compressive strength may be measured at a specifiedtime when the composition is maintained under specified temperature andpressure conditions. Compressive strength can be measured by eitherdestructive or non-destructive methods. The destructive methodphysically tests the strength of treatment fluid samples at variouspoints in time by crushing the samples in a compression-testing machine.The compressive strength is calculated from the failure toad divided bythe cross-sectional area resisting the load and is reported in units ofpound-force per square inch (psi). Non-destructive methods may employ aUCA™ ultrasonic cement analyzer, available from Fann Instrument Company,Houston, Tex. Compressive strength values may be determined inaccordance with API RP 10B-2, Recommended Practice for Testing WellCements, First Edition, July 2005.

By way of example, the cement compositions may develop a 24-hourcompressive strength in the range of from about 50 psi to about 5000psi, alternatively, from about 100 psi to about 4500 psi, oralternatively from about 500 psi to about 4000 psi. In some embodiments,the cement compositions may develop as compressive strength in 24 hoursof at least about 50 psi, at least about 100 psi, at least about 500psi, or more. In sonic embodiments, the compressive strength values maybe determined using destructive or non-destructive methods at atemperature ranging from 100° F. to 200° F.

In sonic embodiments, the cement compositions may have desirablethickening times. Thickening time typically refers to the time a fluid,such as a cement composition, remains in a fluid state capable of beingpumped. A number of different laboratory techniques may be used tomeasure thickening time. A pressurized consistometer, operated inaccordance with the procedure set forth in the aforementioned API RPPractice 10B-2, may be used to measure whether a fluid is in a pumpablefluid state. The thickening time may be the time for the treatment fluidto reach 70 Bc and may be reported as the time to reach 70 Bc. In someembodiments, the cement compositions may have a thickening time ofgreater than about 1 hour, alternatively, greater than about 2 hours,alternatively greater than about 5 hours at 3,000 psi and temperaturesin a range of from about 50° F. to about 400° F., alternatively, in arange of from about 80° F. to about 250° F., and alternatively at atemperature of about 140° F.

As will be appreciated by those of ordinary skill in the art,embodiments of the cement compositions may be used in as variety ofcementing operations, including primary and remedial cementing. In someembodiments, a cement composition may be provided that comprises water,a pozzolan, and hydrated lime. The cement composition may be introducedinto a subterranean formation and allowed to set therein. As usedherein, introducing the cement composition into a subterranean formationincludes introduction into any portion of the subterranean formation,including, without limitation, into a wellbore drilled into thesubterranean formation, into a near wellbore region surrounding thewellbore, or into both.

In some embodiments, a set-delayed cement composition may be providedthat comprises water, a pozzolan, lime, a set retarder, and optionally adispersant The set-delayed cement composition may be stored, forexample, in a vessel or other suitable container. The set-delayed cementcomposition may be permitted to remain in storage for a desired timeperiod. For example, the set-delayed cement composition may remain instorage for a time period of about 1 day or longer. For example, theset-delayed cement composition may remain in storage for a time periodof about 1 day, about 2 days, about 5 days, about 7 days, about 10 days,about 20 days, about 30 days, about 40 days, about 50 days, about 60days, or longer. In some embodiments, the set-delayed cement compositionmay remain in storage for a time period in a range of from about 1 dayto about 7 days or longer. Thereafter, the set-delayed cementcomposition may be activated, for example, by addition of a cement setactivator, introduced into as subterranean formation, and allowed to settherein.

In primary cementing embodiments, for example, an embodiment of a cementcomposition may be introduced into an annular space between a conduitlocated in a wellbore and the walls of a wellbore (and/or a largerconduit in the wellbore), wherein the wellbore penetrates thesubterranean formation. The cement composition may be allowed to set inthe annular space to form an annular sheath of hardened cement. Thecement composition may form a barrier that prevents the migration offluids in the wellbore. The cement composition may also, for example,support the conduit in the wellbore.

In remedial cementing embodiments, a cement composition may be used, forexample, in squeeze-cementing operations or in the placement of cementplugs. By way of example, the composition may be placed in a wellbore toplug an opening (e.g., a void or crack) in the formation, in a gravelpack, in the conduit, in the cement sheath, and/or between the cementsheath and the conduit (e.g., a microannulus).

An embodiment discloses a method of formulating a cement composition.The method may comprise measuring surface area of a group of pozzolans,wherein the smallest measured surface area of the group of pozzolans andthe largest measured surface area of the group of pozzolans vary byabout 50% or greater. The method may further comprise selecting one ormore pozzolans from the group of pozzolans. The method may furthercomprise adding components comprising lime and water to the selected oneor more pozzolans to form a cement composition.

An embodiment discloses a method of cementing in a subterraneanformation. The method may comprise providing a first pozzolan. Themethod may further comprise providing a second pozzolan, wherein thesecond pozzolan comprises pozzolan particles having a surface area thatvaries by about 50% or more from pozzolan particles of the firstpozzolan. The method may further comprise blending the first pozzolanand the second pozzolan to form a blended pozzolan. The method mayfurther comprise adding components lime and water to the blendedpozzolan to form a cement composition. The method may further compriseintroducing the cement composition into the subterranean formation. Themethod may further comprise allowing the cement composition to set inthe subterranean formation.

Referring now to FIG. 1, preparation of a cement composition (which maybe set delayed or non-set delayed) in accordance with exampleembodiments will now be described. FIG. 1 illustrates a system 2 forpreparation of a cement composition and delivery to a wellbore inaccordance with certain embodiments. As shown, the cement compositionmay be mixed in mixing equipment 4, such as a jet mixer, re-circulatingmixer, or a hatch mixer, for example, and then pumped via pumpingequipment 6 to the wellbore. In sonic embodiments, the mixing equipment4 and the pumping equipment 6 may be disposed on one or more cementtrucks as will be apparent to those of ordinary skill in the art. Insome embodiments, a jet mixer may be used, for example, to continuouslymix the lime/settable material with the water as it is being pumped tothe wellbore. In set-delayed embodiments, a re-circulating mixer and/ora batch mixer may be used to mix the set-delayed cement composition, andthe activator may be added to the mixer as a powder prior to pumping thecement composition downhole.

An example technique for placing a cement composition into asubterranean formation will now be described with reference to FIGS. 2and 3. FIG. 2 illustrates surface equipment 10 that may be used inplacement of a cement composition in accordance with certainembodiments. It should be noted that while FIG. 2 generally depicts aland-based operation, those skilled, in the art will readily recognizethat the principles described herein are equally applicable to subseaoperations that employ floating or sea-based platforms and rigs, withoutdeparting from the scope of the disclosure. As illustrated by FIG. 2,the surface equipment 10 may include a cementing unit 12, which mayinclude one or more cement trucks. The cementing unit 12 may includemixing equipment 4 and pumping equipment 6 (e.g., FIG. 1) as will beapparent to those of ordinary skill in the art. The cementing unit 12may pump a cement composition 14 through a feed pipe 16 and to acementing head 18 which conveys the cement composition 14 downhole.

Turning now to FIG. 3, the set-delayed or non-set-delayed pozzolaniccement composition 14 may be placed into a subterranean formation 20 inaccordance with example embodiments. As illustrated, a wellbore 22 maybe drilled into the subterranean formation 20. While wellbore 22 isshown extending, generally vertically into the subterranean formation20, the principles described heroin are also applicable to wellboresthat extend at an angle through the subterranean formation 20, such ashorizontal and slanted wellbores. As illustrated, the wellbore 22comprises walls 24. In the illustrated embodiment, a surface casing 26has been inserted into the wellbore 22. The surface casing 26 may becemented to the walls 24 of the wellbore 22 by cement sheath 28. In theillustrated embodiment, one or more additional conduits (e.g.,intermediate casing, production casing, liners, etc.), shown here aseasing 30 may also be disposed in the wellbore 22. As illustrated, thereis a wellbore annulus 32 formed between the casing 30 and the walls 24of the wellbore 22 and/or the surface easing 26. One or morecentralizers 34 may be attached to the casing 30, for example, tocentralize the casing 30 in the wellbore 22 prior to and during thecementing operation.

With continued reference to FIG. 3, the cement composition 14 may bepumped down the interior of the casing 30. The cement composition 14 maybe allowed to flow down the interior of the casing 30 through thecasing, shoe 42 at the bottom of the casing 30 and up around the casing30 into the wellbore annulus 32. The cement composition 14 may beallowed to set in the wellbore annulus 32, for example, to form a cementsheath that supports and positions the casing 30 in the wellbore 22.While not illustrated, other techniques may also be utilized forintroduction of the pozzolanic cement composition 14. By way of example,reverse circulation techniques may be used that include introducing thecement composition 14 into the subterranean formation 20 by way of thewellbore annulus 32 instead of through the casing 30.

As it is introduced, the cement composition 14 may displace other fluids36, such as drilling fluids and/or spacer fluids that may be present inthe interior of the casing 30 and/or the wellbore annulus 32. At least aportion of the displaced fluids 36 may exit the wellbore annulus 32 viaa flow line 38 and be deposited, for example, in one or more retentionpits 40 (e.g., a mud pit), as shown on FIG. 2. Referring again to FIG.3, a bottom plug 44 may be introduced into the wellbore 22 ahead of thecement composition 14, for example, to separate the cement composition14 from the fluids 36 that may be inside the casing 30 prior tocementing. After the bottom plug 44 reaches the landing collar 46, adiaphragm or other suitable device should rupture to allow thepozzolanic cement composition 14 through the bottom plug 44. In FIG. 3,the bottom plug 44 is shown on the landing collar 46. In the illustratedembodiment, a top plug 48 may be introduced into the wellbore 22 behindthe cement composition 14. The top plug 48 may separate the cementcomposition 14 from a displacement fluid 50 and also push the cementcomposition 14 through the bottom plug 44.

The exemplary cement compositions disclosed herein may directly orindirectly affect one or more components or pieces of equipmentassociated with the preparation, delivery, recapture, recycling, reuse,and/or disposal of the disclosed cement compositions. For example, thedisclosed cement compositions may directly or indirectly affect one ormore mixers, related mixing equipment, mud pits, storage facilities orunits, composition separators, heat exchangers, sensors, gauges, pumps,compressors, and the like used generate, store, monitor, regulate,and/or recondition the exemplary cement compositions. The disclosedcement compositions may also directly or indirectly affect any transportor delivery equipment used to convey the cement compositions to a wellsite or downhole such as, for example, any transport vessels, conduits,pipelines, trucks, tubulars, and/or pipes used to compositionally movethe cement compositions from one location to another, any pumps,compressors, or motors (e.g., topside or downhole) used to drive thecement compositions into motion, any valves or related joints used toregulate the pressure or flow rate of the cement compositions, and anysensors (i.e., pressure and temperature), gauges, and/or combinationsthereof, and the like. The disclosed cement compositions may alsodirectly or indirectly affect the various downhole equipment and toolsthat may come into contact with the cement compositions such as, but notlimited to, wellbore casing, wellbore completion string, insert strings,drill string, coiled tubing, slickline, wireline, drill pipe, drillcollars, mud motors, downhole motors and/or pumps, cement pumps, surfacemounted motors and/or pumps, centralizers, turbolizers scratchers,floats (e.g., shoes, collars, valves, etc.), logging tools and relatedtelemetry equipment, actuators (e.g., electromechanical devices,hydromechanical devices, etc.), sliding sleeves, production sleeves,plugs, screens, filters, flow control devices (e.g., inflow controldevices, autonomous inflow control devices, outflow control devices,etc.), couplings (e.g., electro-hydraulic wet connect, dry connect,inductive coupler, etc.), control lines (e.g., electrical, fiber optic,hydraulic, etc.), surveillance lines, drill bits and reamers, sensors ordistributed sensors, downhole heat exchangers, valves and correspondingactuation devices, tool seals, packers, cement plugs, bridge plugs, andother wellbore isolation devices, or components, and the like.

To facilitate a better understanding of the embodiments, the followingexamples of certain aspects of some embodiments are given. In no wayshould the following examples be read to limit, or define, the entirescope of the disclosure.

EXAMPLES Example 1

Four pumice samples were obtained, with each sample having similarparticle sizes but a different surface area. The pumice samples wereDS-325 grade, available from Hess Pumice Products, Inc., Malad, Id.DS-325 grade is a -325 mesh lightweight aggregate pumice, according tothe mesh measurements the DS-325 grade Pumice should typically have, ad50 particle size distribution of about 12-17 microns, The details ofthe sample properties are described below in Table 1.

TABLE 1 Pumice Sample Analysis Surface Sample Pumice Grade Density(g/mL) Area (m²/g) D₅₀ (μm) 1 DS-325 2.28 4.09 14.0 2 DS-325 2.30 10.2815.4 3 DS-325 2.46 5.90 14.2 4 DS-325 2.51 17.90 12.3

Measurement of the particle sizes show that the mesh estimationapproximates the experimentally determined particle sizes. The particlessize was measured using a Malvern Mastersizer 3000 available fromMalvern Instruments, Worcestershire, United Kingdom. Pumice Samples 3and 4 were used to create two cement composition slurries. ExperimentalSample 1 and Experimental Sample 2 respectively as illustrated in Table2 below.

TABLE 2 Experimental Sample Description Experimental Pumice DensitySurface Area D₅₀ Sample Grade (g/mL) (m²/g) (μm) 1 DS-325 2.46 5.90 14.22 DS-325 2.51 17.90 12.3

With the exception of a suspending agent added to Experimental Sample 1to aid particle suspension, the cement composition slurries wereprepared in the exact same way so that the only major difference betweenthe two samples was the surface area of the pumice. The composition ofthe experimental samples is presented in Table 3 below.

TABLE 3 Experimental Sample Compositional Makeup Component Amount (%bwoP)* Water 60.0 Pumice 100.0 Hydrated Lime 20.0 Dispersant 0.60Retarder 1.25 Co-Retarder 0.50 Weighting Agent 2.0 Suspending Agent**0.035 *% by weight of Pumice; **only used in Experimental Sample 1

The compositions had a density of 13.5 pounds per gallon. The dispersantwas Liquiment® 5581F dispersant available from BASF, Florham Park; N.J.The cement retarder was Micro Matrix® Cement Retarder available fromHalliburton Energy Services, Inc., Houston Tex. The cement co-retarderwas HR®-5 Cement Retarder available from Halliburton Energy Services,Inc., Houston, Tex. The weighting agent was Micromax® FF weight additiveavailable from Halliburton Energy Services, Inc., Houston, Tex. Thesuspending agent was SA-1015™ suspending agent available fromHalliburton Energy Services, Inc., Houston, Tex.

After preparation, the shelf lives of Experimental Sample 1 andExperimental Sample 2 were measured by placing a portion of each sampleinto a sealed container and leaving them at ambient temperature andpressure conditions. The samples were checked daily to measure and toobserve changes. Experimental Sample 2 gelled and became overly viscousat 12 days and required additional dispersant in order to reduce theviscosity. Experimental Sample 1 maintained a pumpable fluid state forgreater than 40 days.

Next, the theological properties of the experimental samples weremeasured using a Model 35A Farm Viscometer and a No. 2 spring with aFann Yield Stress Adapter (FYSA), in accordance with the procedure setforth in API RP Practice 10B-2, Recommended Practice for Testing WellCements. The volume average viscosities (VAV) were calculated using thetorque dial readings and the calibrated factors that convert RPM toshear rate and dial readings to shear stress. Table 4 tabulates theapparent viscosity of the composition at 100 rpm versus the compositionage.

TABLE 4 Volume Average Viscosity at 100 RPM (Centipoise) AgeExperimental Sample 1 Experimental Sample 2 Day 0 351 975 Day 3 273 1566Day 11 234 2795 Day 12 — >3000 Day 21 247 —* Day 30 273 — Day 42 429 —*— means that the viscosity was not measured for that day.

As is shown by the data, the high surface area slurry (ExperimentalSample 2) exhibited a consistently higher volume average viscosity thanthat of the lower surface area slurry. Over 11 days the VAV_(100rpm) ofExperimental Sample 2 averaged 1779 cP, whereas the VAN_(100rpm) ofExperimental Sample 1 averaged only 286 cP.

Lastly, both experimental samples were activated. Each experimentalsample was split into two portions and an activator added to each. Oneactivator was a 2% active liquid additive combination of sodiumhexainetaphosphate (SHMP) and sodium sulfate, the other activator was a10% active calcium chloride. After activation, the samples were pouredinto 2″ by 4″ brass cylinders and cured for 24 hours at 140° F. in awaterbath. After the samples were cured, the destructive compressivestrength was measured by using a mechanical press to crush the samplesin accordance with the procedure set forth in API RP Practice 10B-2,Recommended Practice for Testing Well Cements. The results are presentedin Table 5 below.

TABLE 5 Compressive Strength Measurement Experimental Pumice SurfaceCompressive Sample Area (m²/g) Activator Strength (psi) 1 5.90 2%SHMP/Na₂SO₄ 551 2 17.90 2% SHMP/Na₂SO₄ 934 1 5.90 10% CaCl₂ 0 2 17.9010% CaCl₂ 390

These results are an average of three experiments. The results indicatethat pumice particles with a higher surface area are more reactive thanpumice particles with lower surface areas. Experimental Sample 2therefore had a higher compressive strength than Experimental Sample 1.However, Experimental Sample 2 also exhibited a faster gelation and ahigher viscosity than Experimental Sample 1 which may have negativeimplications in set-delayed embodiments or in any embodiment requiringthe cement composition to remain in a pumpable fluid state for anextended period.

Example 2

Experimental Samples 3 and 4 were prepared to create two cementcomposition slurries. Experimental Sample 3 and Experimental Sample 4utilized Pumice comprising not just different surface areas, butdifferent particles sizes as well in order to demonstrate thecontribution particle size may have with respect to cement properties.The characteristics of the samples are illustrated in Table 6 below.

TABLE 6 Experimental Sample Description Experimental Pumice DensitySurface Area D₅₀ Sample Grade (g/mL) (m²/g) (μm) 3 DS-200 2.39 4.32 23.04 DS-325 2.51 17.90 12.3

The cement composition slurries were prepared in the exact same way sothat the only major difference between the two samples was the surfacearea and the particle size of the pumice. The composition of theexperimental samples is presented in Table 7 below.

TABLE 7 Experimental Sample Compositional Makeup Component Amount (%bwoP)* Water 60.0 Pumice 100.0 Hydrated Lime 20.0 Dispersant 0.60Retarder 1.25 Co-Retarder 0.50 Weighting Agent 2.0 *% by weight ofPumice

The compositions had a density of 13.5 pounds per gallon. The dispersantwas Liquiment® 5581F dispersant available from BASF, Florham Park, N.J.The cement retarder was Micro Matrix® Cement Retarder available fromHalliburton Energy Services, Inc., Houston, Tex., the cement co-retarderwas HR®-5 Cement Retarder available from Halliburton Energy Services,Inc., Houston, Tex. The weighting agent was Micromax® FF weight additiveavailable from Halliburton Energy Services, Inc., Houston, Tex.

After preparation, the shell lives of Experimental Sample 3 andExperimental Sample 4 were measured by placing a portion of each sampleinto a sealed container and leaving them at ambient temperature andpressure conditions. The samples were checked daily to measure toobserve changes. Experimental Sample 4 gelled and became overly viscousat 10 days and required additional dispersant in order to reduce theviscosity. Experimental Sample 3 maintained a pumpable fluid state forgreater than 17 days.

Next, the rheological properties of the experimental samples weremeasured using a Model 35A Farm Viscometer and a No. 2 spring with aFarm Yield Stress Adapter (FYSA), in accordance with the procedure setforth in API RP Practice 10B-2, Recommended Practice for Testing WellCements. The volume average viscosities (VAV) were calculated using thetorque dial readings and the calibrated factors that convert RPM toshear rate and dial readings to shear stress. Table 8 tabulates theapparent viscosity of the composition at 100 rpm versus the compositionage.

TABLE 8 Volume Average Viscosity at 100 RPM (Centipoise) AgeExperimental Sample 3 Experimental Sample 4 Day 0 364 975 Day 5 568 1859Day 11 546 2795 Day 18 754 —* *— means that the viscosity was notmeasured for that day.

As is shown by the data, the low surface area—large particle size slurry(Experimental Sample 3) exhibited a consistently lower volume averageviscosity than that of the high surface area—small particle size slurry.Over 11 days the VAV_(100rpm) of Experimental Sample 3 averaged 493 cP,whereas the VAV_(100rpm) of Experimental Sample 4 averaged 1876 cP.

Lastly, both experimental samples were activated. Each experimentalsample was split into two portions and an activator added to each. Oneactivator was a 2% active iquid additive combination of sodiumhexametaphosphate (SHMP) and sodium sulfate, the other activator was a10% active calcium chloride. After activation, the samples were pouredinto 2″ by 4″ brass cylinders and cured for 24 hours at 140° F. in awater bath. After the samples were cured, the destructive compressivestrength was measured by using a mechanical press to crush the samplesin accordance with the procedure set forth in API RP Practice 10B-2,Recommended Practice for Testing Well Cements. The results are presentedin Table 9 below.

TABLE 9 Compressive Strength Measurements Pumice Particle SurfaceExperimental Size Area Compressive Sample D₅₀ (μm) (m²/g) ActivatorStrength (psi) 3 23.0 4.32 2% SHMP/Na₂SO₄ 193 4 12.3 17.90 2%SHMP/Na₂SO₄ 934 3 23.0 4.32 10% CaCl₂ ~50 4 12.3 17.90 10% CaCl₂ 390

These results are an average of three experiments. The results indicatethat pumice particles with a smaller particle sizes and higher surfaceareas are more reactive than pumice particles with larger particle sizesand lower surface areas. Experimental Sample 4 therefore had a highercompressive strength than Experimental Sample 3. However, ExperimentalSample 4 also exhibited a faster gelation and a higher viscosity thanExperimental Sample 3 which may have negative implications in setdelayed embodiments or in any embodiment requiring the cementcomposition to remain in a pumpable fluid state for an extended period,

It should be understood that the compositions and methods are describedin terms of “comprising,” “containing,” or “including” variouscomponents or steps, the compositions and methods can also “consistessentially of” or “consist of” the various components and steps.Moreover, the indefinite articles “a” or “an,” as used in the claims,are defined herein to mean one or more than one of the element that itintroduces,

For the sake of brevity, only certain ranges are explicitly disclosedherein, However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as, rangesfrom any lower limit max be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues even if not explicitly recited. Thus, every point or individualvalue may serve as its own lower or upper limit combined with any otherpoint or individual value or any other lower or upper limit, to recite arange not explicitly recited.

Therefore, the present embodiments are well adapted to attain the endsand advantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only and the maybe modified and practiced in different but equivalent manners apparentto those skilled in the art having the benefit of the teachings herein.Although individual embodiments are discussed, all combinations of allthose embodiments are covered by the disclosure. Furthermore, nolimitations are intended to the details of construction or design hereinshown, other than as described in the claims below. Also, the terms inthe claims have their plain, ordinary meaning unless otherwiseexplicitly and clearly defined by the patentee. It is therefore evidentthat the particular illustrative embodiments disclosed, above may bealtered or modified and all such variations are considered within thescope and spirit of those embodiments. If there is any conflict in theusages of a word or term in this specification and one or more patent(s)or other documents that may be incorporated herein by reference, thedefinitions that are consistent with this specification should beadopted.

What is claimed is:
 1. A method of cementing in a subterranean formationcomprising: providing a first pozzolan; providing a second pozzolan,wherein the second pozzolan comprises pozzolan particles having asurface area that varies by about 50% or more from pozzolan particles ofthe first pozzolan; blending the first pozzolan and the second pozzolanto form a blended pozzolan; adding components comprising lime and waterto the blended pozzolan to form a cement composition, whereincementitious components present in the cement composition comprisepozzolans comprising the first pozzolan and the second pozzolan withoutany additional cementitious components other than the pozzolans thathydraulically set in the presence of the water; introducing the cementcomposition into the subterranean formation; and allowing the cementcomposition to set in the subterranean formation.
 2. A method accordingto claim 1 wherein the first pozzolan and the second pozzolan areindividually selected from the group consisting of: fly ash, silicafume, metakaolin, pumice, and any combination thereof.
 3. A methodaccording to claim 1 wherein the cement composition further comprises aset retarder, wherein the set retarder comprises at least one retarderselected from the group consisting of a phosphonic acid, a phosphonicacid derivative, a lignosulfonate, a salt, an organic acid, a cellulosederivate, a synthetic co- or ter-polymer comprising sulfonate andcarboxylic acid groups, a borate compound, and any combination thereof.4. A method according to claim 1 wherein the cement composition furthercomprises at least one dispersant selected from the group consisting ofa sulfonated-formaldehyde-based dispersant, a polycarboxylated etherdispersant, and a combination thereof.
 5. A method according to claim 1wherein the cement composition further comprises a cement set activatorselected from the group consisting of calcium chloride, triethanolamine,sodium silicate, zinc formate, calcium acetate, sodium hydroxide, sodiumsulfate, nanosilica, sodium hexametaphosphate, and any combinationsthereof.
 6. A method according to claim 1 wherein the cement compositionremains in a pumpable fluid state for at least about 7 days prior tointroducing the cement composition into the subterranean formation.
 7. Amethod according to claim 1 wherein the cement composition is introducedinto the subterranean formation by a process comprising pumping thecement composition through a conduit and into a wellbore annulus.
 8. Amethod according to claim 1 wherein a ratio of the first pozzolan to thesecond pozzolan in the blended pozzolan is from about 80:20 to about20:80.
 9. A method according to claim 1 wherein the second pozzolan hasa particle size that varies by about 10% or less than a particle size ofthe first pozzolan.
 10. A method according to claim 1 wherein the firstpozzolan and the second pozzolan each individually have a particle sizeof from about 10 microns to about 25 microns.
 11. A system for cementingin a subterranean formation comprising: a first pozzolan, a secondpozzolan; wherein the second pozzolan comprises pozzolan particleshaving a surface area that varies by about 50% or more from pozzolanparticles of the first pozzolan, lime; and water; mixing equipmentcapable of mixing a cement composition, wherein the cement compositioncomprises the first pozzolan, the second pozzolan, the lime, and thewater, wherein cementitious components present in the cement compositioncomprise pozzolans comprising the first pozzolan and the second pozzolanwithout any additional cementitious components other than the pozzolansthat hydraulically set in the presence of water; and pumping equipmentcapable of pumping the cement composition formed by mixing the firstpozzolan, the second pozzolan, the lime, and the water.
 12. The systemaccording to claim 11 wherein the first pozzolan and the second pozzolanare individually selected from the group consisting of: fly ash, silicafume, metakaolin, pumice, and any combination thereof.
 13. The systemaccording to claim 11 further comprising a set retarder, wherein the setretarder comprises at least one retarder selected from the groupconsisting of a phosphonic acid, a phosphonic acid derivative, alignosulfonate, a salt, an organic acid, a cellulose derivate, asynthetic co- or ter-polymer comprising sulfonate and carboxylic acidgroups, a borate compound, and any combination thereof.
 14. A method ofcementing in a subterranean formation comprising: providing a firstpozzolan; providing a second pozzolan, wherein the second pozzolancomprises pozzolan particles having a surface area that varies by about50% or more from pozzolan particles of the first pozzolan; blending thefirst pozzolan and the second pozzolan to form a blended pozzolan;adding components comprising lime and water to the blended pozzolan toform a cement composition, wherein cementitious components present inthe cement composition comprise pozzolans comprising the first pozzolanand the second pozzolan without any additional cementitious componentsother than the pozzolans that hydraulically set in the presence of thewater; storing the cement composition in a pumpable fluid state forabout a day or longer; introducing the cement composition into thesubterranean formation; and allowing the cement composition to set inthe subterranean formation.
 15. A method according to claim 14 whereinthe first pozzolan and the second pozzolan are individually selectedfrom the group consisting of: fly ash, silica fume, metakaolin, pumice,and any combination thereof.
 16. A method according to claim 14 whereinthe cement composition further comprises a set retarder, wherein the setretarder comprises at least one retarder selected from the groupconsisting of a phosphonic acid, a phosphonic acid derivative, alignosulfonate, a salt, an organic acid, a cellulose derivate, asynthetic co- or ter-polymer comprising sulfonate and carboxylic acidgroups, a borate compound, and any combination thereof.
 17. A methodaccording to claim 14 wherein the cement composition further comprisesat least one dispersant selected from the group consisting of asulfonated-formaldehyde-based dispersant, a polycarboxylated etherdispersant, and a combination thereof.
 18. A method according to claim14 further comprising added at least on cement set activator to thecement composition after the step of storing, wherein the cement setactivator is selected from the group consisting of calcium chloride,triethanolamine, sodium silicate, zinc formate, calcium acetate, sodiumhydroxide, sodium sulfate, nanosilica, sodium hexametaphosphate, and anycombinations thereof.
 19. A method according to claim 14 wherein thecement composition is stored in the pumpable fluid state for at leastabout 7 days prior to introducing the cement composition into thesubterranean formation.
 20. A method according to claim 14 wherein aratio of the first pozzolan to the second pozzolan in the blendedpozzolan is from about 80:20 to about 20:80.
 21. A method according toclaim 14 wherein the second pozzolan has a particle size that varies byabout 10% or less than a particle size of the first pozzolan.